Semiconductor switches are widely used as basic circuit components and provide system level design flexibility. For example, in audio applications, semiconductor switches are used to select a single audio output from multiple audio source channels. Another example of a typical semiconductor switch application is illustrated in FIG. 1. In cellular phone 100 semiconductor switch circuit 108 is used to share signal jack 114 with USB port 102 and audio driver 104. By sharing single signal jack 114, the number of external signal interfaces on cellphone 100 can be reduced. Control circuit 106 asserts a control signal that activates internal logic 112 to change the state of single pole double throw (SPDT) switch 110. Signal jack 114 can then be coupled to a computer via a USB cable, or to a loudspeaker via an audio cable depending on the mode of operation of cellphone 100.
FIG. 2 illustrates a conventional semiconductor switch 116 using a CMOS transmission gate 120. Transmission gate 120 has NMOS transistor N1 and PMOS transistor P1 that couples signal IN with signal OUT depending on the state of signal CONTROL. Signal CONTROL is coupled to gate of NMOS transistor N1 directly and to the gate of PMOS transistor P1 via inverter I1. During operation, when signal CONTROL is high, both NMOS transistor N1 and PMOS transistor P1 are ON. When signal CONTROL is low, both NMOS transistor N1 and PMOS transistor P1 are OFF. The on-resistance of the CMOS transmission gate, however, depends on the bias point of transistors N1 and P1. Generally, the on-resistance of CMOS transmission gate 120 will vary according to the power supply voltage (not shown) and the voltages of signals IN and OUT. When the power supply voltage is low, the variation of the on-resistance of CMOS transmission gate 120 can vary considerably with respect to signals IN and OUT.
In some audio applications, such as headphone and speaker drivers, the variation in on-resistance of a CMOS switch can introduce undesirable distortion in an audio signal. The audio signal suffers even more distortion if signals IN and OUT coupled to CMOS switch 120 are operated outside of the power supply range of the switch 116. For example, if the signals IN and OUT are at 0V and have an appreciable peak-to-peak amplitude, the resulting audio signal may be significantly distorted. Some prior art solutions, such as the constant gate drive MOS analog switch described in U.S. Pat. No. 6,154,085, address the issue of resistance variation by providing a constant gate drive to a switch transistor. The prior art circuit, however, consumes power and does not provide a constant gate drive for signals outside of the power supply range, thereby resulting in the distortion of moderate to large audio signals biased at 0V.
In the field of power supplies, what are needed are power efficient semiconductor switches that provide low on-resistance variation.